Since polyunsaturated fatty acids are essential fatty acids, the amount ingested for satisfying the required amount from a nutritional viewpoint has been discussed. At present, however, since the ingested amount is sufficient, attention has focused on serum lipid lowering effects, resulting in a situation in which emphasis is placed on the ratio of unsaturated fatty acids to saturated fatty acids of ingested fats. On the other hand, there are two representatives series of unsaturated fatty acids, namely omega-3 and omega-6 (omega indicates the number of carbon atoms from the terminal methyl group of the fatty acid to the carbon atom at which the first double bond is located). Recently, a growing emphasis has come to be placed on the ratio of these omega-6 fatty acids to omega-3 fatty acids.
Although it has been confirmed that various fatty acids such as omega-6 fatty acids, including linoleic acid, dihomo-.gamma.-linolenic acid and arachidonic acid, and omega-3 fatty acids, including .alpha.-linolenic acid, eicosapentaenoic acid and docosahexaenoic acid, exhibit different physiological actions, at the same time, what is important is that these two series of unsaturated fatty acids mutually have a potent effect on the physiological action of the other, as well as that both of these series of fatty acids cannot be biosynthesized in the body, both series are not interchangeable, and the ratio of omega-3 and omega-6 unsaturated fatty acids in the body reflects that in the diet.
In view of these circumstances, Japanese Unexamined Patent Publication No. 3-53869 discloses a food in which the fatty acid composition therein is adjusted so that the ratio of omega-3 fatty acids to omega-6 fatty acids is 1:1 to 1:5. In the revision of Japanese nutritional requirements of 1994 (Japanese Ministry of Health and Welfare, 5th Revision of Japanese Nutritional Requirements, pp. 56-58, 1994), it is stated that the preferable ratio of omega-6 unsaturated fatty acids to omega-3 unsaturated fatty acids is 4:1. However, it is difficult during the daily life to ingest only foods in which the ratio of omega-6 and omega-3 fatty acids is adjusted, and is essentially difficult to constantly keep in mind the ingestion ratio of omega-6 and omega-3 unsaturated fatty acids.
In addition, since the Japanese diet has been Westernized recently, there has been a considerable increase in the opportunities to consume meals made primarily of meat resulting in an increase in the ingestion of omega-6 fatty acids in comparison with omega-3 fatty acids. Due to this trend, there has been a dramatic increase in the mortality rate due to arteriosclerotic diseases such as myocardial infarction and cerebral thrombosis. In order to improve this situation, foods and nutritional supplements have been developed to which have been added omega-3 unsaturated fatty acids such as eicosapentaenoic acid and docosahexaenoic acid concentrated to high concentrations. However, in considering actual dietary habits, it is virtually impossible to consume only one type of fatty acid. In particular, it is dangerous to ingest large amount of only one of these types of unsaturated fatty acids in consideration of the physiological functions of omega-3 and omega-6 unsaturated fatty acids.
For instance, examples of metabolic disorders thought to occur due to ingestion of large amounts of omega-6 unsaturated fatty acids include: (1) disturbance in the balance of eicosanoid production (promotion of thrombus formation, arteriosclerosis and allergic reactions), (2) accelerated gallstone formation, (3) promotion of cancer cell growth (including breast cancer and colon cancer), and (4) depressed immunity and reduced phagocyte function. In addition, examples of metabolic disorders thought to occur accompanying excessive ingestion of omega-3 unsaturated fatty acids (and particularly fish oil containing eicosapentaenoic acid and docosahexaenoic acid) include: (1) myocardial necrosis, (2) liver disorders and decreased liver function, (3) increased sensitivity to catecholamines, and (4) myocardial lipidosis caused by long-chain monoenic acids, increased bleeding time, and greater susceptibility to hemorrhaging and clotting difficulties due to reduced platelet levels.
As has been described above, when referring to unsaturated fatty acids, it is not appropriate to refer to omega-6 series or omega-3 series fatty acids alone. In order to maintain homeostasis of the body and prevent disease, the ingestion ratio of omega-6 and omega-3 series unsaturated fatty acids among the unsaturated fatty acids ingested must be balanced. In addition, those omega-6 unsaturated fatty acids which can be obtained from a normal diet mainly comprise linoleic acid, while the majority of omega-3 unsaturated fatty acids are .alpha.-linolenic acid or eicosapentaenoic acid and docosahexaenoic acid from fish oil.
Since it is known that eicosapentaenoic acid and docosahexaenoic acid inhibit conversion from linoleic acid to dihomo-.gamma.-linolenic acid and arachidonic acid (precursors of omega-6 eicosanoids), the balance of these unsaturated fatty acids has an effect on the fatty acid composition of the body, which effect is greater than their actual consumption ratio. Thus, it is extremely difficult to determine the ingestion ratio and its amounts in consideration of dynamics in the body. Therefore, there was a strong desire to develop a safe substance that suitably adjusts the ratio of omega-6 unsaturated fatty acids and omega-3 unsaturated fatty acids in the body in order to maintain homeostasis of the body as well as prevent disease.